1. Field of the Invention
This invention relates generally to microelectronic packaging and interconnection, and more particularly, to apparatuses and methods in which dual ball bonds, metal bumps or TAB bonds are formed by laser, but not conventional heating, compression or acoustic, means.
2. Brief Description of the Prior Art
Wire bonding has been one of the most important techniques for making microelectronic interconnections. Two types of wire bonding methods are widely used in IC package assembly operations: the gold ball bonding method and the wedge bonding method. The gold ball bonding method, despite being so named, involves the formation of a wire loop with a ball bond at one end but a wedge bond at the other end. This bonding method is only suitable for gold or slightly doped gold wires. On the other hand, the wedge bonding method delivers wedge bonds at both ends of the wire loop and is mostly used for aluminum wire bonding, although gold wires are occasionally used. Generally speaking, between these two methods, the gold ball bonding method delivers better throughput and shorter wire loop length while the aluminum wedge bonding method gives relatively finer bonding pitch.
In either of the above wire bonding methods, mechanical means, e.g., compression or ultrasound, or the combinations thereof, are used to form the ball bonds and the wedge bonds. In addition, thermal energy must be provided to the substrate during a gold ball bonding process. Thus, depending on the specific mechanical means involved, there are two well-established gold ball bonding techniques: thermocompression bonding and thermosonic bonding. In both techniques, thermal energy is supplied by either conventional (e.g., resistive) heating means or laser heating to keep the substrate at an elevated temperature during the formation of the bonds to ensure adequate bonding strength.
For example, U.S. Pat. No. 4,529,115, issued to T. Renshaw et al. and entitled "Thermally Assisted Ultrasonic Welding Apparatus and Process," teaches a conventional thermally assisted ultrasonic bonding device including an electrical resistance coil wrapped around the bonding tip. A limitation of this device is that the thermal mass being heated is very large compared to the specific bonding area. Moreover, an elevated temperature of the thermal mass may not only pose a safety hazard to the operator but also be detrimental to the operability of the adjacent or integral heat sensitive components.
As another example, U.S. Pat. No. 4,534,811, issued to N. Aisnlie et al. and entitled "Apparatus for Thermo Bonding Surfaces," describes a conventional laser-assisted ultrasonic bonding device including a laser and a hollow ultrasonic bonding tip. The combination of heat supplied by the laser and the sonic energy supplied by the bonding tip offers the ability to dynamically provide bonding energy in a short pulse to a limited bonding area. However, the laser component taught in this reference is expensive, complex and incompatible with many modern bonding processes. Excessive ultrasonic forces may also be detrimental to the integrity of electronic components embedded beneath the bonding area. For example, bond pad "cratering" may be caused by either excessive ultrasonic forces or compression.
As further examples, U.S. Pat. No. 4,718,591, issued to W. Hill and entitled "Wire Bonder With Open Center of Motion," discloses an ultrasonic bonding machine having an open center mounting that permits optimum, substantially linear motion of a bonding tip, whereas U.S. Pat. No. 4,598,853, issued to W. Hill and entitled "Open Center Flexural Pivot Wire Bonding Head," discloses a flexural pivot structure useful in an apparatus for bonding thin wire leads in microelectronic circuits.
Attempts have also been made to use thermal energy alone to bond electrical or electronic components. An example is U.S. Pat. No. 3,718,968, issued to Sims et al. and entitled "Method for Connecting a Wire to a Component," which discloses a bonding method including the following steps: deforming the end of a wire by heating it above its melting point with a laser beam and permitting it to solidify into a sphere; placing the sphere in contact with the component; and heating the sphere and the bonding area of the component by a laser beam above their respective melting points to provide a fusion weld between the wire and the component. Essentially, this reference teaches that, to avoid vaporization of the end of the wire before the bonding area is melted, the sphere must be made as large as necessary to provide sufficient reduction in its area-to-volume ratio, and that the duration of laser heating must be long enough to raise the temperatures of both the sphere and the component bonding area above their respective melting points but not above the vaporization point of the sphere. These requirements, however, are over-simplified ones, and are at best a necessary, but not sufficient, condition for successful laser-driven bonding.
Having thus described prior-art technology disclosed in several issued patents, a schematic representation of a conventional, "generic" gold ball bonding apparatus and method is depicted in FIGS. 1A-1E. In FIG. 1A, a wire made of gold or specially doped gold is passed through a bonding tip or "capillary" 12, which is an essential part of the conventional ball bonding apparatus 14. The capillary 12 itself may or may not be heated. An electric spark 16 is used to melt the end of the wire 10 to form a ball 18; this is known as the "electric flame-off" or "EFO" step. The newly formed gold ball 18 is brought to the close proximity of a bond pad 20 on an IC chip (i.e., the substrate) 22, which is typically heated to 300-400.degree. C. A somewhat lower substrate temperature may be used if sonic energy is applied in conjunction with substrate heating.
Next, as depicted in FIG. 1B, the capillary 12 presses the ball against the bond pad 20 during a short ultrasonic pulse to form a gold ball bond 24 in the form of a "nail head," the bottom of which is bonded to the bond pad 20. This is commonly known as the "ball bonding" step.
In FIG. 1C, the capillary 12 is shown lifted from the substrate 22 to allow the newly formed gold ball bond 24 to unreel the wire 10 through the capillary 12.
In FIG. 1D, the capillary 12 is shown moved away from the chip 22 and repositioned above a bonding finger (or lead frame) 26 of an IC package 28; as a result, a wire loop 30 is formed.
Finally, as depicted in FIG. 1E, a wedge bond 32 is formed by shearing the gold wire on the bonding finger 26 of the IC package 28; that is, the capillary 12 presses the gold wire against the bonding finger 26 and moves essentially horizontally away from the bonding finger 26. The capillary 12 is then raised while a small length of wire is unreeled. A prior-art electromagnetic wire clamp (not shown) is actuated to break the wire at the neck of the wedge 32, leaving a flattened tail adhering to the bonding finger 26. Thus, the IC bond pad 20 and a bonding finger 26 are electrically connected through the gold wire loop 30, one end of which is connected to the IC bond pad 20 via a ball bond 24 while the other end is connected to the bonding finger 26 via a wedge bond 32. This wire bonding process can be repeated for the connection of the remaining bond pads of the IC chip to the corresponding bonding fingers of the IC package, creating a completed and functional IC device.
The aforementioned ball bonding apparatus and method works fairly well for wires made of soft metals such as gold. However, many problems do occur. First, to allow the wire to pass through its center, the conventional capillary is usually quite bulky, setting a limit on the finest bonding pitch achievable with the gold ball bonding technique. Second, the EFO step often damages the capillary and significantly shortens the useful life of the capillary. Third, to ensure good contact between the wire and the bonding surface at a wedge bond, the capillary generally has to make a low-entry-angle motion relative to the bonding surface. This low angle necessarily entails a longer wire loop and longer "interpad" distance between the bond pad of the chip and the bond pad or bonding finger of the package than if the wire loop could come down at the package bonding plane at a greater entry angle, i.e., closer to 90.degree.. Fourth, because the IC chips are subjected to substrate heating and mechanical stress caused by compression or ultrasonic forces, various forms of IC chip damages may occur, resulting in a lower overall yield. If wires made of metals possessing greater hardness and higher melting points, such as copper or palladium, were used, the overall yield would be even lower, because higher substrate temperature and more mechanical energy would be required to achieve bonding. In fact, such IC damage and yield problems have practically prevented fine copper or palladium wires from being widely used in wire-bonded microelectronic packages, even though these metals are both electrically better (e.g. greater electromigration resistance) and mechanically stronger than gold. This limitation of conventional wire bonding is well documented in the literature.
Note that, compared to the above ball bonding method, the conventional wedge bonding method requires an even greater interpad distance because of the low entry angles dictated by the wedge bonds at both the chip bond pad and the package bond pad.
The aforementioned conventional gold ball bonding method can be modified to produce gold bumps on an IC chip. Such gold bumps may be used in attaching the IC chip directly to a circuit board or an IC package (either a ceramic package, plastic package, or a flexible circuit board) via the flip-chip method, or in other advanced packaging assembly applications such as MCM (multichip module), MEMS (mechanical-electronic microsensor), and hard disk assembly.
FIGS. 2A-2C depict this conventional "metal stub" method for gold bump formation. In FIG. 2A, a gold ball 18 is formed by the aforementioned EFO process. The newly formed ball 18 is then brought in contact with a bond pad 20 of an IC chip 22; see FIG. 2B. A ball bond 24 in the form of a nail head is produced on the bond pad by the aforementioned ball bonding step. As shown in FIG. 2C, the capillary 12 is raised to unreel a small length of wire 10. A prior-art wire clamp 16 is then electromagnetically actuated to break the wire 10 right above the nail head 24, leaving a gold bump having a short and sharp tail 34. The above steps depicted in FIGS. 2A-2C may be repeated to generate gold bumps on all of the bond pads of the IC chip so that the bond pads can be directly attached to a circuit board or other connecting structures via the bumps.
Similar to the above formation of ball bonds between an IC chip and gold wires in the conventional ball bonding method, the substrate in the aforesaid prior-art gold bumping method is generally kept at an elevated temperature and subjected to vicious mechanical compression and shear to ensure acceptable bond strength. Thus, the IC damage and yield problems cited above for the conventional ball bonding method persist in this conventional gold bumping method as well. Furthermore, because the finished gold bumps have uncontrolled sharp tails due to the nature of metal fracture, additional planarization processes are generally required to flatten these sharp tails before the chip can be attached to a circuit board or another structure via, e.g., the flip-chip process. Finally, the average height of finished gold bumps produced by this conventional gold bumping process is usually several mils tall, making this process unsuitable for today's miniaturized IC packaging assemblies.
Another microelectronic packaging technique that has showed great promises is the tape-automated-bonding (commonly dubbed TAB) assembly process. TAB was originally developed as a highly automatic technique for packaging large volume, low I/O devices, but has since been applied to high I/O devices (more than 300 I/O connections) as well. Typically, the TAB process involves the use of thermal compression bonding to bond silicon chips to metal (e.g., copper) strips deposited and patterned on a polymer (e.g., polyimide or Mylar) tape. Thus, TAB technology is generally considered as both a chip connection method and a first-level packaging technique.
A typical example of a TAB tape 40 currently used in package industry is shown in FIG. 3A. This TAB tape 40 consists of laminated and patterned copper leads 42 glued to prepunched high-temperature polymer film 44. The copper leads 42 may also be coated with an electroplated solder layer to optimize the TAB bonding operation and provide better corrosion resistance. This solder electroplating process requires that all copper leads be shorted to electrode contact points. This adversely affects the usable area on the tape, and additionally, may require a special cutting or punching operation to delete common connections where it is desirable to test Inner Lead Bonding (ILB) bonded devices prior to Outer Lead Bonding (OLB).
In principle, to bond IC chips to a TAB tape, bump contacts can be made on either the IC chip or the TAB tape. However, formation of bumped TAB tapes is rather complicated and expensive. This makes the use of bumped chip the method of choice for TAB bonding.
An example of a conventional bumped chip TAB bonding process is illustrated in FIG. 3B. Through a series of time-consuming and labor-intensive processes, including thin film deposition, photoresist coating, photolithography alignment and exposure, development of photoresist, solder electroplating, post-plating etching, etc. Solder bumps 46 are generated on the IC chips 48 while the chips 48 are still in wafer form. Subsequently, the bumped wafer is examined and sawed to liberate each individual IC chip 48. These IC chips 48 are then transferred onto a sticky or wax layer 50 attached to a polymer tape carrier 52. These bumped IC chips 48 are affixed to the copper leads 42 of the TAB tape 40 as follows. First, the tips of the copper leads 42 are visually aligned to the corresponding solder bump contacts 46 of the IC chip 48 via an optical alignment system. Then, the copper leads 42 are compressed down against the solder bump contacts 46 and heated by a resistive heating block 54, with or without ultrasound, forming bonding between each copper lead 42 and its corresponding solder bump contact 46. Alternatively, as shown in FIG. 3C, the resistive heating block 54 can be replaced by a hollow capillary 56 to accommodate a low-power laser beam 58 which provides heat needed for the lead-to-bump bonding.
As an example, U.S. Pat. No. 4,893,742, issued to P. Bullock and entitled "Ultrasonic Laser Soldering," teaches a "flux-less" TAB laser soldering apparatus, where an elongated ultrasonically vibratable capillary is employed to press the wire down against its pad and to receive and guide an optical fiber through which a beam of laser energy is fed coaxially through the capillary to heat the wire end.
In addition to possibly excessive heating and mechanical stress, a severe limitation of the conventional TAB bonding technique is that the unbumped areas of the copper leads and the IC chip must generally be masked with a patterned photoresist layer prior to solder plating. This requires additional care, process steps, and process time. Furthermore, either the TAB tape or the IC chip, or both, are exposed to undesirable wet chemicals during the photolithographic masking process and the electrochemical plating process. Thus, the conventional TAB bonding method has been regarded by many skilled in the art as a messy, complex, and time-consuming process. Such complexity of the conventional TAB process also entails a significantly higher operational cost than wire bonding methods, preventing the TAB technique from being widely accepted by the cost-conscious microelectronic packaging industry.
The aforesaid problems, i.e., substrate damage caused by thermal and mechanical means, reduction in the overall yield, damage to the capillary, needs for photolithographic masking and electroplating, etc., will become worse and worse as the size of IC chips moves towards miniaturization; that is, I/O pads and transistors shrink in size and increase in density. Hence, there is a need in the art for a microwelding apparatus and process which allows finer pitch bonds and bumps without the need for photolithographic masking, electrochemical plating, or excessive heating, compression or ultrasonic vibration.
All of the patents mentioned above are hereby incorporated by reference for purposes of additional disclosure.